The ability to see and recognize objects is crucial for human behavior. To support this important skill, our brains contain regions involved in processing the visual aspects of the world surrounding us. Some of these brain regions are involved in perceiving ecologically relevant categories such as faces, bodies, places or written words. While faces or bodies are seen frequently from birth and shape the visual brain early in life, extensive experience with written words only starts when children learn how to read.
This raises the question - what may be the initial role of brain regions that during childhood learn to process written words and eventually support the important skill of reading? This question is particularly interesting because in evolutionary terms reading is a relatively recent cultural invention. In other words, it is unlikely that humans are born with a brain region dedicated to processing letters and words.
We addressed this question in our recent paper. To this end, we collected functional magnetic resonance imaging (fMRI) data in school-age children. In a team effort, we gathered over the course of several years multiple datasets per child. This massive set of measurements allowed us to track the development of individual children’s brains over time. When the study started, our child participants were between 5-12 years old (on average 8.5 years old). By the end of the study, most of them were teens (13-17 years old). During each fMRI session children viewed images of visual categories including words, faces, objects, body parts and places.
To identify the brain regions that are involved in processing a certain visual category, we can compare the brain responses when participants see images belonging to one category, such as faces to other categories (such as words, numbers, limbs, cars, guitars, houses & corridors). Using this approach, we identified brain regions involved in processing faces, words, limbs, and places in each child. Since each child had participated in our experiment several times, we were able track how these regions develop over time. For example, we identified the region that processes written words in a child at age 7 and then again at age 10. We noticed that when we identified this part of their brain that processes written words it was largely in the same location in the brain across years - in a cortical fold called the occipital temporal sulcus. But we also noticed that it was larger when the child was 10 or 11 than when they were 6 or 7 years old. In fact, in most children, regions involved in processing words were larger when they were teens than where they were children. Similarly, regions involved in processing faces were typically larger in the teens than in childhood. This was consistent with our predictions as teens are better at reading and recognizing faces than children, and theories of brain development suggest that as you become better at these tasks there are more neurons in your brain that become specialized for processing these stimuli. However, to our surprise, we found that regions that are involved in processing limbs shrank in the same children. That is, in most participants these regions were actually larger in young children compared to teenagers. This was not what we had expected, as there were no theories of visual development that predicted that there will be less cortical territory devoted to processing a category as you grow.
We were also curious to know what brain territories that were later involved in processing and perceiving words or faces were processing earlier in childhood. Since we were making measurements in the same child’s brain over time, we were able to ‘go back in time’ and look at this growing region – that is later on involved in reading words – and examine what it was processing before, when the child was young.
Surprisingly, our results revealed that before this region was involved in processing written words, it was involved in processing limbs. Similarly, we found that the growing part of the region that is involved in processing faces, was also involved in processing limbs earlier during childhood. Thus, our results suggest that a visual region’s preferred category may change from one to another during childhood development – in other words, we can think about it as being recycled.
This finding raises a few new questions: First, why does this cortical recycling occur during childhood? Second, why are regions that process limbs larger in younger children compared to teenagers? We hypothesize that these neural changes in responsiveness may reflect changes in what children frequently look at and the tasks they perform at different ages. For instance, it has been shown that toddlers spend a lot of time looking at hands. This may be because they rely strongly on gestures and often look at their own hands as they figure out how manipulate objects and toys. We speculate that brain regions involved in processing limbs may therefore be larger in young children than in teens, who look less at hands but more at written words and faces, as they spent a lot of time reading and interacting with others in school. So it may be that recycling in visual brain regions may reflect adjustment to changing visual demands during childhood. We are excited to test these hypotheses in future research.